Flubs
Here's an idea of how far I will go for something as trivial as this list. I had to watch Plan 9 not once, but TWICE to get these errors down! The word "obviously" comes up a lot in this list, and only because I feel it is necessary to point out how obvious some of the flaws are.

In case you couldn't tell [translation: in case you're blinder than a bat!], the errors are numbered, and after the numbers is the approximate time into the movie the error BEGINS to occur. Some errors last only a split-second, while others last several minutes.

[0:00:14] Jump cut

[0:00:31] Criswell redundancy: "Future events such as these will affect you in the future."

[0:01:16] Criswell is obviously reading a cue card.

[0:02:45] The priest at the funeral never opens his mouth to say a word.

[0:12:21] "You mean the kind [of flying saucers] from up there?" No, you dumb***k, the OTHER kind of flying saucers!!!!

[0:12:27] Jeff describes the flying saucers as "cigar-shaped!" (For those of you who haven't seen the movie--they're saucer-shaped!)

[0:13:13] A strong wind from the flying saucer's exhaust is strong enough to blow Jeff and Paula to the ground, but not strong enough to move their hair!

[0:13:35] Someone trips over a tombstone and causes it to wobble.

[0:14:28] The zombi-fied old man is obviously a stand-in for Bela Lugosi, as a) he has more hair than Bela did, b) he's much taller, c) he's thinner, and d) holds his cape over his face to [very badly] hide the fact that he's NOT Bela! [This stand-in will from here on be referred to as "Dr. Tom."]

[0:15:13] Zombie Vampira flinches when Inspector Clay fires at her.

[0:16:07] The detective scratches himself with the barrel of his loaded gun.

[0:16:50] The priest at Clay's funeral is obviously reading from a cue card.

[0:18:10] The flying saucer is flying low. REALLY low.

[0:19:10] The rockets seem to all explode in exactly the same place in the sky, with the power and longevity of exploding firecrackers.

[0:20:01] There’s a small mysterious black piece of fuzz in the "sky."